Myotonic dystrophy (DM) is a multisystem disease and the most common form of muscular dystrophy in adults. In 1992, one form of DM was shown to be caused by an expanded CTG repeat in the 3' untranslated region of the myotonin protein kinase gene (DMPK) on chromosome 19. Although multiple theories attempt to explain how the CTG expansion causes the broad spectrum of clinical features in DM, there is no consensus about how this mutation, which does not alter the protein coding region of a gene, affects cellular function. We have identified a five-generation family (MN1) with a genetically distinct form of myotonic dystrophy. Affected members have the characteristic features of DM (myotonia, proximal and distal limb weakness, frontal balding, cataracts, and cardiac arrhythmias) but do not have the chromosome 19 mutation. We have mapped the disease locus (DM2) for the MN1 family to a small region of chromosome 3 (Nature Genetics 19:196- 198). This proposal outlines a strategy to identify and characterize the DM2 locus. Understanding what is common to chromosome 19 DM (now designated DM1 by the DM consortium) and DM2 at the molecular level should shed light on the mechanisms responsible for the broad constellation of clinical features present in both diseases. Our specific aims are: 1) to develop a high-resolution map of the DM2 region (0.5-1.0 cM) using haplotype and linkage disequilibrium analysis of 29 DM2/PROMM families from Minnesota and Germany; 2) to identify the expressed genes and repeat motifs in the region and prioritize candidates based on homology and expression patterns; 3) to identify the DM2 mutation; 4) to characterize the DM2 gene and investigate whether or not the pathogenic molecular changes found in DM2 are part of a common pathway also affected in DM1; 5) to determine whether molecular changes affecting RNA splicing, CUG binding proteins, and apamin receptors are similar to those found in DM1.